Hydraulic Conductivity in a Piñon-Juniper Woodland: Influence of Vegetation

shrublands (Elkins et al., 1986; Lyford and Qashu, 1969; Wainwright et al., 2000), mesquite rangelands (Wood In semiarid environments, vegetation affects surface runoff either and Blackburn, 1981), and piñon-juniper rangelands in by altering surface characteristics (e.g., surface roughness, litter absorption) or subsurface characteristics (e.g., hydraulic conductivity). the USA (Roundy et al., 1978). Similar findings have Previous observations of runoff within a piñon-juniper [Pinus edulis been reported from other parts of the world. Examples Englem. and Juniperus monosperma (Englem.) Sarg.] woodland led are Australia, where studies were performed in both us to hypothesize that hydraulic conductivity differs between vegetamulga woodlands (Greene, 1992) and arid shrublands tion types. Using ponded and tension infiltrometers, we measured (Dunkerley, 2000a); Niger, in tiger bush (Bromley et saturated (Ks) and unsaturated [K(h )] hydraulic conductivity at three al., 1997); and Spain, in semiarid shrublands (Cerda et levels of a nested hierarchy: the patch (canopy and intercanopy), the al., 1998). In other studies, differences in infiltrability unit (juniper canopy, piñon canopy, vegetated intercanopy, and bare have been found within the intercanopy, between areas intercanopy), and the intercanopy locus (grass, biological soil crust, exhibiting differing degrees of herbaceous cover (Wilbare spot). Differences were smaller than expected and generally not cox et al., 1988). Similarly, Wood and Blackburn (1981) significant. Canopy and intercanopy Ks values were comparable with the exception of a small number of exceedingly high readings under found higher infiltration rates for mid-grass than for the juniper canopy—a difference we attribute to higher surface macroshort-grass areas. And in Spain, Cerda (1997) reported porosity beneath juniper canopies. The unsaturated hydraulic conducthat infiltration rates under the grass species Stipa tenativity, K(h ), values were higher for canopy soils than for intercanopy cissima were almost double those for adjacent bare soils, although differences were small. At the unit level, the only ground. significant differences were for K(h ) between juniper or piñon canoEnhanced infiltrability under vegetation canopies pies vs. bare interspaces. Median K values for vegetated intercanopy may be due to a number of factors, including textural areas were intermediate between but not significantly different from differences resulting from rain splash or trapping of those for canopies and bare areas. There were no significant differeolian sands by vegetation (Parsons et al., 1992); higher ences between grass, biological soil crust, and bare spots within the organic-matter content of the soil under vegetation; proherbaceous intercanopy area. Overall, the observed differences in K between canopy and intercanopy patches do not account for differtection of the soil surface by leaf litter; enhanced aggreences in runoff observed previously. gation; and a more developed network of macropores (Dunkerley, 2000a). Intercanopy soils often have low infiltrability that could be a result of the relatively harsher microclimate (Breshears et al., 1998), comparaI semiarid landscapes, there is generally an inverse tively small inputs of organic matter, and the developrelationship between vegetation cover and overland ment of an erosion pavement or soil crust layer (Blackflow. In other words, all other factors being equal, the burn et al., 1975). Within the intercanopy zone itself, more vegetation, the less overland flow. This may occur soil infiltrability has been observed to vary with differeither as a result of enhanced soil infiltrability, for which ences in surface cover. The biological soil crusts that hydraulic conductivity (K) is a direct indicator, or modiare common in arid and semiarid landscapes modify soil fied surface characteristics (e.g., a change in surface hydrology and stability in these regions (Belnap and roughness or surface storage), such that water has Lange, 2001). The relative effect of these modifications greater opportunity to infiltrate into the soil. In this has been demonstrated to be strongly influenced by soil paper we examine the relationship between one of these texture: studies show that biological soil crusts reduce factors, soil infiltrability, and vegetation cover in a pithe infiltrability of very sandy soils, whereas they enñon-juniper community in New Mexico. hance or have little effect on the infiltrability of more Soil infiltrability is closely linked to vegetation cover. fine-textured soils (Warren, 2001). The literature is replete with examples of the positive On the basis of the extensive literature establishing relationship between vegetation cover and soil infiltrathe strong linkage between vegetation cover and numerbility—showing, in particular, that the infiltrability of ous hydrologic characteristics—including infiltration, soils under shrub canopies is generally higher than that runoff, and erosion—we propose that in semiarid landof intercanopy soils. Significantly higher infiltrability scapes vegetation cover can serve as the criterion for has been documented for shrub canopy soils in sagebrush rangelands (Blackburn, 1975; Johnson and Gorthe identification of “hydrologic functional units” (Wildon, 1988; Pierson et al., 1994; Seyfried, 1991), creosote cox and Breshears, 1995). This may be a useful approach for dealing with the strong scale-dependent relationship B.P. Wilcox, Rangeland Ecology and Management, Texas A&M for runoff in semiarid landscapes (Seyfried and Wilcox, Univ., College Station, TX 77843; D.D. Breshears, Environmental 1995; Wilcox et al., 2003). At larger scales, for example, Dynamics and Spatial Analysis, Mail Stop J495, Los Alamos National runoff per unit area dramatically decreases with increasLab., Los Alamos, NM 87545; H.J. Turin, Environmental Technology, Mail Stop J534, Los Alamos National Lab., Los Alamos, NM 87545. Received 6 Mar. 2002. *Corresponding author ([email protected]). Abbreviations: Ks, saturated hydraulic conductivity; K(h ), unsaturated hydraulic conductivity. Published in Soil Sci. Soc. Am. J. 67:1243–1249 (2003).